In the printing industry, it is often desirable to move a premanufactured graphic image from one location to another. Modern facsimile system technology provides the capability of scanning an image at one location and transmitting the resulting data to another location for printing.
The original graphic image may be comprised of a number of components assembled on a photographic medium, e.g. film or paper, as they are to appear on a final printed page.
The various components that might be present in a graphic image are:
1. Type, primarily comprising alphanumeric characters. PA1 2. Line art, typically line illustration, and rules composed of black and white linework, or patterns of various complexity. PA1 3. Halftone illustrations representative of continuous tone images which reproduce the gray levels of an image with patterns of dots of varying area. PA1 1. The original medium to be scanned, transmitted, and recorded. This is usually a photographic medium of very high contrast which can be film based either with black elements on a transparent background (positive), or clear elements on a black background (negative), or paper based (reflection) with black elements on a white reflective background. PA1 2. The scanning subsystem, consisting of a light source to illuminate the image, a photosensing device to capture the transmitted or reflected light, and a single level threshold (usually adjustable) that simply puts out binary white bits for light levels above the threshold and black bits for light levels beneath. PA1 3. The transmission system, which is typically some form of communications link between the scanning subsystem and the output/recorder subsystem. This can be as simple as a direct wire connection, or as complex as satellite transmission with various compression/decompression schemes applied to the data stream. PA1 4. The output/recorder subsystem, typically a binary imaging system capable of exposing or not exposing each picture element on a piece of film or paper in an exact one to one relationship with the original scanned image.
A typical prior art facsimile system causes an illumination spot to scan the prepared graphic image to produce a stream of binary (either black or white) picture elements which are then transmitted to a receiver/recorder for subsequent printing. Such facsimile systems are well known and generally include the following:
Facsimile systems suitable for use in the graphic arts/printing industry encounter far more stringent requirements than relatively low resolution facsimile systems used in a typical office environment. For example, a graphic arts facsimile system, is required to capture the finest serifs on very small type and faithfully reproduce all of the gray levels represented in halftone dots which can contain picture elements of 0.001" or less. That is, in order to maintain gray level fidelity and color balance, all halftone dots must be reproduced at precisely the same size as in the original image.
The faithful reproduction of graphic images, and particularly halftone dots, has presented a significant challenge, due in part to the limitations of the scanning subsystem. Some of those limitations are attributable to the basically Gaussian shape of the sampling spot. That is, the Gaussian shape of the spot permits neighboring picture elements, around the element being sampled, to influence and distort the binary black/white decision made with respect to the sampled picture element. The result of this distortion is the loss of information needed to faithfully reproduce the original image, most noticeable in halftone dot growth or dropout. For example, scanner limitations typically cause 95% halftone dots to grow and appear totally black and 5% halftone dots to dropout and appear totally white. This shifts the gray level or color balance of the image and causes loss of subtle detail.
Prior efforts have been made to eliminate these and other sources of distortion; e.g., see U.S. Pat. No 4,501,016 and U.S. Pat. No. 4,926,267. Other patents of interest are U.S. Pat. No. 4,597,009, U.S. Pat. No. 4,723,173, U.S. Pat. No. 4,742,399, U.S. Pat. No. 4,829,587, U.S. Pat. No. 4,903,143. U.S. Pat. No. 4,905,294, U.S. Pat. No. 4,908,875, and U.S. Pat. No. 4,953,114.